Ball screw mechanism

ABSTRACT

Between two mutually adjoining load balls  5 , there is interposed a retaining piece  10  having two concave surfaces  11  which are respectively opposed to the two adjoining load balls  5 . The retaining piece  10  has an outside diameter dimension equal to or more than 0.5 times the outside diameter dimension of the load ball  5 . Further, the retaining piece  10  has an outside diameter dimension which, when the retaining piece  10  passes through a screw groove circulation passage and a tube circulation passage, prevents the retaining piece  10  from touching the inner walls of these screw groove circulation passage and tube circulation passage, or has an outside diameter dimension which is equal to or less than 0.9 times the outside diameter dimension of the load ball. Accordingly, it is possible to provide a ball screw mechanism which can prevent a retaining piece from interfering with the inner walls of circulation passages to circulate load balls stably to thereby prevent variations in torque as well as prevent the wear of the retaining piece to thereby enhance the durability thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 10/737,811 filed Dec. 18,2003, which is a divisional of application Ser. No. 10/254,672 filedSep. 26, 2002, now U.S. Pat. No. 6,742,408, which is a continuation ofapplication Ser. No. 09/693,980, now abandoned; the disclosure of theseapplications is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a ball screw mechanism in which, evenwhen a retaining piece is interposed between two mutually adjoining loadballs, decrease in the number of load balls can be controlled as much aspossible to thereby prevent the load balls from degrading the loadcapacity and rigidity thereof, can reduce friction between the loadballs and retaining piece to thereby enhance the circulating performanceof the retaining piece, and can prevent the load balls from buttingagainst each other to thereby prevent degradation in the operationefficiency thereof, deterioration in the quality of sounds generated,and the friction and damage of the load balls.

Conventionally, in a ball screw mechanism, as shown in FIG. 9, on theouter peripheral surface of a screw shaft 1 and on the inner peripheralsurface of a nut 2, there are respectively formed spiral-shaped screwgrooves 3 and 4 which disposed opposed to each other and, in a screwgroove circulation passage defined by the two screw grooves 3 and 4,there are disposed a large number of load balls 5 in such a manner thatthey are free to roll. In case where the screw shaft 1 and nut 2 arerotated with respect to each other to thereby move one of them in theaxial direction thereof, the screw shaft 1 and nut 2 are smoothly movedin a spiral manner with respect to each other.

In such ball screw mechanism, the load balls 5 are arranged denselywithin the screw grooves 3 and 4 and they roll in the same directionwithin the individual screw grooves 3 and 4; and, in the rollingmovements thereof, at contact points between the two mutually adjoiningload balls 5 and 5, the load balls 5 rolling in the mutually oppositedirections are contacted with each other to thereby interfere with theirmutual rolling movements, with the result that the load balls 5 and 5are caused to slip at the contact points. This interferes with the freerolling movements of the load balls 5, degrades the operation efficiencyof the load balls 5, gives rise to friction and damage in the load balls5, causes variations in the torque of the load balls 5, and swells thenoises that are produced by the load balls 5.

To cope with these problems, as shown in FIG. 10, between the mutuallyadjoining load balls 5, there are respectively interposed spacer balls 6each having a diameter smaller by several tens .μ. than the load balls5. That is, such interposition of the spacer balls 6 prevents the loadballs 5 against slippage, improves the operation efficiency of the loadballs 5, and reduces the friction and damage of the load balls 5 tothereby prevent variations in the torque.

However, in the ball screw mechanism shown in FIG. 10, while the loadballs 5 are, for example, ten in number, the spacer balls 6 are also,for example, ten in number. Therefore, when compared with the ball screwmechanism shown in FIG. 9, a clearance between the two adjoining loadballs 5 is large and the load balls 5 are reduced in number down toabout one half, which decreases the load capacity of the ball screwmechanism and also degrades the rigidity thereof.

In view of the above problems, there is also known a structure in which,as shown in FIG. 11, between every two mutually adjoining load balls 5,there is interposed a retaining piece 10 having two concave surfaces 11respectively facing the two load balls 5. According to this structure,while the load balls 5 are in contact with the concave surfaces 11 ofthe retaining piece 10, they are allowed to circulate well within thespiral-shaped screw grooves 3 and 4. Therefore, when compared with theconventional structure using spacer balls, the spacer, that is, theretaining piece 10 can be made thinner, which makes it possible tocontrol reduction in the number of load balls and thus avoid degradationin the load capacity and rigidity of the ball screw mechanism.

However, in the ball screw mechanism shown in FIG. 11, since therelation between the outside dimension of the retaining piece 10 and theoutside dimension of the load ball 5 is not always considered properly,there is a fear that, when the retaining piece 10 passes through thespiral-shaped screw groove circulation passage and tube circulationpassage, it can interfere with the inner walls of these circulationpassages. Therefore, such interference makes it difficult for the loadballs 5 to circulate stably, thereby giving rise to generation ofvariations in the torque, or causing the retaining piece 10 to wear.

SUMMARY OF THE INVENTION

The present invention aims at eliminating the drawbacks found in theabove-mentioned conventional ball screw mechanisms. Accordingly, it isan object of the invention to provide a ball screw mechanism whichprevents the retaining piece from interfering with the inner walls ofthe circulation passages to thereby allow the load balls to circulatestably, thereby preventing not only variations in the torque but alsothe wear of the retaining piece for enhancement of the durability of theretaining piece.

In attaining the above object, according to a first aspect of theinvention, there is provided a ball screw mechanism structured suchthat, on the outer peripheral surface of a screw shaft and on the innerperipheral surface of a nut, there are respectively formed mutuallyfacing spiral-shaped screw grooves, there are rollably disposed a largenumber of load balls in a spiral-shaped screw groove circulation passageformed by these two screw grooves, and, to the screw groove circulationpassage, there is continuously connected a return circulation passagethrough which load balls taken out from the screw groove circulationpassage can be returned again into the screw groove circulation passage,wherein, between every two mutually adjoining ones of the large numberof load balls, there is interposed a retaining piece having two concavesurfaces respectively facing the two mutually adjoining load balls, theretaining piece has an outside diameter dimension equal to or more than0.5 times the outside diameter dimension of the load ball, and theretaining piece has an outside diameter dimension which, when theretaining piece passes through the screw groove circulation passage andtube circulation passage, prevents the retaining piece from touching theinner walls of the screw groove circulation passage and tube circulationpassage.

Also, according to a second aspect of the invention, there is provided aball screw mechanism structured such that, on the outer peripheralsurface of a screw shaft and on the inner peripheral surface of a nut,there are respectively formed mutually facing spiral-shaped screwgrooves, there are rollably disposed a large number of load balls in aspiral-shaped screw groove circulation passage formed by these two screwgrooves, and, to the screw groove circulation passage, there iscontinuously connected a return circulation passage through which loadballs taken out from the screw groove circulation passage can bereturned again into the screw groove circulation passage, wherein,between every two mutually adjoining ones of the large number of loadballs, there is interposed a retaining piece having two concave surfacesrespectively facing the two mutually adjoining load balls, and theretaining piece has an outside diameter dimension in the range of 0.5 to0.9 times the outside diameter dimension of the load ball.

As described above, according to the invention, as a first condition,the retaining piece has an outside diameter dimension equal to or morethan 0.5 times the outside diameter dimension of the load ball. This isbasically because, in case where the retaining piece has an outsidediameter dimension less than half the outside diameter dimension of theload ball, the retaining piece cannot be lifted up from between the twomutually adjoining load balls, which disables the retaining piece tofulfill the expected function thereof.

Also, as a second condition, the retaining piece has an outside diameterdimension which, when the retaining piece passes through the screwgroove circulation passage and tube circulation passage, prevents theretaining piece from touching the inner walls of these screw groovecirculation passage and tube circulation passage, or has an outsidediameter dimension which is equal to or less than 0.9 times the outsidediameter dimension of the load ball. This is basically because, tocirculate the load balls stably, the concave surface holding allowancefor the load balls (the steel ball holding allowance) maybe preferablyset large, which increases the outside diameter of the retaining pieceaccordingly; but, in case where the concave surface holding allowance isexcessively large, the retaining piece interferes with the inner wallsof the screw groove circulation passage and return circulation passagehaving various curvatures, thereby degrading the operation efficiency ofthe retaining piece. Under these circumstances, to balance them witheach other, the outside diameter of the retaining piece is set as largeas possible on one side but, on the other hand, the outside diameter ofthe retaining piece is controlled to such a degree that it does notinterfere with the inner walls of the circulation passages.

As described above, according to the invention, since, between every twomutually adjoining load balls, there is interposed a retaining piecehaving two concave surfaces respectively facing the two load balls, evenin case where a retaining piece between two mutually adjoining loadballs, the number of load balls can be reduced as many as possiblewithout degrading the load capacity and rigidity of the ball screwmechanism; and, friction between the load balls and retaining pieces canbe reduced as much as possible, so that not only the circulatingperformance of the retaining piece can be enhanced, but also the loadballs can be prevented from butting each other, thereby preventing thedegraded operation efficiency of the load balls, generation of noises bythe load balls, the deteriorated quality of sounds generated by the loadballs, and the friction and damage of the load balls.

In addition, because the relation between the outside diameter dimensionof the retaining piece and the outside diameter dimension of the loadballs is properly considered, the interference of the retaining pieceswith the inner walls of the circulation passages can be prevented,thereby allowing the load balls to circulate stably. This can preventvariations in torque as well as can prevent the wear of the retainingpieces to thereby enhance the durability thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a typical view of a ball screw mechanism according to anembodiment of the invention;

FIG. 2 is a typical view of a state of a retaining piece and its twoadjoining load balls when there exist clearances respectively betweenthe retaining piece and two load balls;

FIG. 3 is a typical view the shape of the side surfaces of the retainingpiece;

FIG. 4(a) is a typical view of a state of the retaining piece and twoload balls when the outside dimension (ds) of the retaining piece isequal to or more than 0.5 times the outside dimension (Dw) of the loadball, and FIG. 4(b) is a typical view of a state of the retaining pieceand two load balls when the outside dimension (ds) of the retainingpiece is equal to or less than 0.5 times the outside dimension (Dw) ofthe load ball;

FIG. 5 is a typical view of a modification of the above embodiment of aretaining piece according to the invention;

FIG. 6 is a graphical representation of the test results of the torquecharacteristic of a conventional ball screw mechanism when the outsidediameter dimension of a retaining piece is 0.95 times the outsidediameter dimension of a load ball;

FIG. 7 is a graphical representation of the test results of the torquecharacteristic of a ball screw mechanism according to the invention whenthe outside diameter dimension of a retaining piece is 0.9 times theoutside diameter dimension of a load ball;

FIG. 8(a) is an explanatory view of the principle of a ball screwmechanism according to an embodiment of the invention, and FIG. 8(b) isa section view of a retaining piece employed in the embodiment;

FIG. 9 is a side view of a first conventional ball screw mechanism;

FIG. 10 is a side view of a second conventional ball screw mechanism;and,

FIG. 11 is a section view of a third conventional ball screw mechanism.

DETAILED DESCRIPTION OF THE INVENTION

Now, description will be given below of the preferred embodiments of aball screw mechanism according to the invention with reference to theaccompanying drawings.

Prior to description of the preferred embodiments of a ball screwmechanism according to the invention, the number of load balls, thenumber of retaining pieces and the clearance relation between the loadballs and retaining pieces will be discussed with reference to FIG. 8.

Now, FIG. 8(a) is an explanatory view of the principle of a ball screwmechanism according to an embodiment of the invention, and FIG. 8(b) isa section view of a retaining piece employed in the embodiment.

The numbers of load balls 5 and retaining pieces 10 are respectively setin the following manner that, as shown in FIG. 8 (a), when it is assumedthat all the balls 5 and all the retaining pieces 10 inserted into aspiral-shaped circulation passage formed by screw grooves 3 and 4 aregathered to one side, a clearance formed between a load ball (LEAD-B)positioned at the head and a retaining piece (TAIL-S) positioned at thetail is defined as a total clearance; and, when it is assumed that thetotal clearance (S1) is larger than zero (that is, S1>0) and thetail-positioned retaining piece (TAIL-S) is removed, a clearance (S2)between the head load ball (LEAD-B) and the tail load ball (TAIL-B) issmaller than 0.8 times the diameter (ds, see FIG. 8 (b)) of theretaining piece (that is, S2<0.8×ds).

As described above, the distance (S1) of the total clearance of thecirculation passage is set such that S1>0 and, when one retaining piece(TAIL-S) is removed, the clearance (S2) between the head load ball(LEAD-B) and the tail load ball (TAIL-B) is set such that S2<0.8×ds.Therefore, there are no possibilities that, because the clearance withinthe circulation passage is excessively large, the retaining pieces 10can be caused to fall down and that, since the clearance within thecirculation passage is excessively small, the load balls 5 and retainingpieces 10 are rubbed against each other to thereby give rise to the pooroperation of the load balls 5 and retaining pieces 10: that is, the twokinds of clearances (S1, S2) within the circulation passage are setproperly. Thanks to this, there is no fear that the retaining pieces 10can fall down further than an angle of 60°, which makes it possible tomaintain the proper operating performance of the load balls 5 andretaining pieces 10.

Next, in the present embodiment, the outside dimension and shape of theretaining piece are defined with above-mentioned conditions taken intoaccount.

Now, FIG. 1 is a typical view of a ball screw mechanism according to thepresent embodiment of the invention; FIG. 2 is a typical view of a stateof a retaining piece and two load balls when there exist clearancesrespectively between the retaining piece and two load balls; FIG. 3 is atypical view of the shape of the side surfaces of the retaining piece;FIG. 4(a) is a typical view of a state of the retaining piece and twoload balls when the outside dimension (ds) of the retaining piece isequal to or larger than 0.5 times the outside dimension (Dw) of the loadball; FIG. 4(b) is a typical view of a state of the retaining piece andtwo load balls when the outside dimension (ds) of the retaining piece isequal to or smaller than 0.5 times the outside dimension (Dw) of theload ball; and, FIG. 5 is a typical view of a modification of theretaining piece according to the second embodiment.

In the present embodiment, as a first condition, the retaining piece 10has an outside dimension which is equal to or larger than 0.5 times theoutside dimension (Dw) of the load ball 5.

The reason why the first condition is established is as follows: thatis, in a ball screw mechanism with retaining pieces contained therein,since the total clearance exists in the above-mentioned manner, all theretaining pieces 10 are not restricted completely between theirrespective adjoining load balls 5 but some of the retaining pieces 10,as shown in FIG. 2, are held in a free state a state which there existsclearances respectively between a retaining piece and two adjoining loadballs) within the circulation passage; and, to return the retainingpiece 10 from the free state to the normal state in which the retainingpiece 10 is tightly held by and between its adjoining load balls 5, itis necessary that, as shown in FIG. 4 (a), the retaining piece 10 has anoutside diameter dimension equal to or more than 0.5 times the outsidediameter dimension Dw. In other words, when the load balls 5 andretaining piece 10 are arranged within the circulation passage in orderof load ball 5-retaining piece 10-load ball 5, if the height of theretaining piece 10 does not exist above a line which connects togetherthe centers of the two loadballs 5, as shown in FIG. 4(b), there doesnot operate a force which lifts the retaining piece 10 up to its normalposition, so that the retaining piece 10 is unable to fulfill itsfunction.

Next, in the present embodiment, as a second condition, the retainingpiece 10 has an outside diameter dimension which, when the retainingpiece 10 passes through a screw groove circulation passage and a tubecirculation passage, prevents the retaining piece 10 from touching theinner walls of these screw groove circulation passage and tubecirculation passage, or has an outside diameter dimension which is equalto or less than 0.9 times the outside diameter dimension of the loadball 5.

(A) As a first reason for the second condition, to secure a concavesurface holding allowance for the load ball 5 (a steel ball holdingallowance, FIG. 3), it is necessary that the outside diameter of theretaining piece 10 is set as large as possible.

That is, to circulate the retaining piece 10 stably in a state where itis held by and between its adjoining load balls 5, it is necessary thatthe concave surface holding allowance for the load ball 5 (the steelball holding allowance, FIG. 3) is set large. However, when the concavesurface holding allowance (the steel ball holding allowance) is setlarge, the outside diameter of the retaining piece 10 increases.

(B) As a second reason for the second condition, it is necessary toprevent the retaining piece 10 from interfering with the inner walls ofthe circulation passages which have various curvatures.

That is, in the case of a ball screw mechanism of a tube type, there arepresent a screw groove circulation passage for receiving loads and atube circulation passage (a return circulation passage) for returningload balls. The screw groove circulation passage has a given curvatureand, at the same time, the tube circulation passage, as shown in FIG. 1,has given curvatures respectively in a bent portion 12 and a ballscoop-up portion 13. And, it is necessary to prevent the retaining piece10 from interfering with the inner walls of the circulation passageshaving various curvatures. For example, as there is shown a continuoustrack of the retaining piece 10 in FIG. 1, it is necessary for theretaining piece 10 to pass through the ball scoop-up portion 13 withoutinterfering with the inner walls thereof.

As described above, to circulate the load balls 5 stably, the concavesurface holding allowance for the load ball 5 (the steel ball holdingallowance, FIG. 3) may be preferably set large and, in this case, theoutside diameter of the retaining piece 10 is set large accordingly.However, in case where the outside diameter of the retaining piece 10 isexcessively large, the retaining piece 10 interferes with the innerwalls of the screw groove circulation passage and return circulationpassage having various curvatures, which degrades the operationefficiency of the retaining piece 10. Under these circumstances, tobalance them with each other, the outside diameter of the retainingpiece 10 is set as large as possible on one side but, on the other hand,it is controlled such that the retaining piece 10 does not interferewith the inner walls of the circulation passages. That is, the secondcondition is defined such that the retaining piece 10 has an outsidediameter dimension which, when the retaining piece 10 passes through ascrew groove circulation passage and a tube circulation passage,prevents the retaining piece 10 from touching the inner walls of thesescrew groove circulation passage and tube circulation passage, or has anoutside diameter dimension which is equal to or less than 0.9 times theoutside diameter dimension of the load ball 5.

Next, as shown in FIGS. 1 to 4, referring to the shape of the retainingpiece 10, the retaining piece 10 includes two concave surfaces 11respectively facing the two adjoining load balls 5, while the outerperipheral surface of the retaining piece 10 in the diameter directionthereof is set flat. Thanks to this shape, the retaining piece 10 isallowed to pass through the screw groove circulation passage and tubecirculation passage without interfering with the inner walls of thesecirculation passages.

Also, referring to the shape of the retaining piece 10, the retainingpiece 10, as shown in FIG. 5, the diameter-direction outer peripheralsurface thereof may also be formed as a triangular-shaped concave shape.In this case, there can be obtained the same effect as the outsidediameter of the retaining piece 10 is reduced, so that while the concavesurface holding allowance (the steel ball holding allowance) is setlarge, it is possible to prevent the retaining piece 10 from interferingwith the inner walls of the bent portion 12 and ball scoop-up portion 13of the tube circulation passage.

Further, as shown in FIG. 1, the bend R of the bent portion 12 of thetube circulation passage may also be set in the following manner.That is, R≈(BCD−Dw)/2where (≈ is the Japanese symbol having the same meaning as the morecommon English symbol ≈), BCD expresses a load ball pitch circlediameter, Dw expresses a load ball outside diameter, and (BCD−Dw)expresses a groove bottom diameter. In this manner, by setting the bendR (that is, a radius curvature) of the bent portion 12 of the tubecirculation passage relatively large, the retaining piece 10 can beprevented from interfering with the bent portion 12, which makes itpossible to circulate the retaining piece 10 stably.

By the way, in case where the distance between the load balls 5 is setlarge, when the balls 5 pass through the bent portion of the tubecirculation passage, there is provided the same tendency as the outsidediameter of the retaining piece 10 is set large. Here, by setting thedistance between the load balls 5 to be equal to or less than 1.3 timesthe outside diameter of the load ball 5, the outside diameter of theretaining piece 10 can be set in the above-mentioned range, so that theabove-mentioned effect can be expected.

By the way, the present invention is not limited to the above-mentionedembodiment but there are possible various changes and modifications.

EMBODIMENT

Now, FIG. 1 shows an inserted state of a retaining piece 10 at a sectionthereof substantially perpendicular to the axial direction thereof whenthe retaining piece 10 is inserted into a ball screw mechanism in whicha load ball (steel ball) diameter is 3.175 mm, a ball screw axialdiameter is Φ 32 mm, and a lead is 5 mm. In this case, the retainingpiece 10 has an outside diameter 90% of the outside diameter of the loadball 5, while the distance between load balls 5 is 115% of the outsidediameter of the load ball 5. As can be seen from FIG. 1, the retainingpiece 10 is prevented from interfering with the inner walls of the screwgroove circulation passage having a given curvature and the inner wallsof the bent portion 12 and ball scoop-up portion 13 of the tubecirculation passage. In the case of this state, test results show thatthere is no problem as to the actual torque characteristic and thedurability of the retaining piece.

Also, FIG. 6 is a graphical representation of the test results of thetorque characteristic of a conventional ball screw mechanism when theoutside diameter dimension of a retaining piece is 0.95 times theoutside diameter dimension of a load ball. In this case, since theretaining piece 10 interferes with the bent portion 12 of the tubecirculation passage, there is found an increase (beard) in the torque ata cycle where the retaining piece 10 passes through the bent portion 12.

Further, FIG. 7 is a graphical representation of the test results of thetorque characteristic of a ball screw mechanism according to theinvention when the outside diameter dimension of a retaining piece is0.9 times the outside diameter dimension of a load ball. In this case,the retaining piece 10 does not interfere with the bent portion 12 ofthe tube circulation passage, nor there is caused an increase (beard) inthe torque when the retaining piece 10 passes through the bent portion12. That is, it can be seen that the torque characteristic of thepresent ball screw mechanism is improved.

As has been described heretofore, according to the invention, since,between every two mutually adjoining load balls, there is interposed aretaining piece which has two concave surfaces respectively facing thesetwo load balls, even in case where a retaining piece is interposedbetween its two mutually adjoining load balls, reduction in the numberof load balls can be controlled as much as possible to thereby preventdegradation in the load capacity and rigidity of the ball screwmechanism; and, friction between the load balls and retaining pieces canbe reduced as much as possible to thereby enhance the circulationperformance of the retaining pieces. Also, the load balls are preventedfrom butting against each other, which can prevent the degradedoperation efficiency of the load balls, generation of noises,deterioration in the quality of sounds generated, and the friction anddamage of the load balls.

In addition, because the relation between the outside diameter dimensionof the retaining piece and the outside diameter dimension of the loadballs is properly considered, the interference of the retaining pieceswith the inner walls of the circulation passages can be prevented tothereby circulate the load balls stably. This can prevent variations intorque as well as can prevent the wear of the retaining pieces tothereby enhance the durability thereof.

While there has been described in connection with the preferredembodiment of the invention, it will be obvious to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the invention, and it is aimed, therefore, to cover inthe appended claim all such changes and modifications as fall within thetrue spirit and scope of the invention.

1. A ball screw mechanism comprising: a screw shaft having a spiral-shaped screw groove on its outer peripheral surface; a nut member having a spiral-shaped screw groove on its inner peripheral surface mutually facing said spiral-shaped screw groove of said screw shaft; a plurality of load balls disposed in a spiral-shaped screw groove circulation passage formed by said mutually facing screw grooves and also in a return circulation passage which is formed in said nut member and is continuously connected to said spiral-shaped screw groove circulation passage, wherein said load balls taken out from said screw groove circulation passage can be returned again into said screw groove circulation passage through said return circulation passage; a plurality of retaining pieces each interposed between every two mutually adjoining ones of said load balls, said retaining piece having two concave surfaces respectively facing said two load balls and also an outside diameter dimension in the range from 0.5 to 0.9 times the outside diameter dimension of said load ball.
 2. (canceled)
 3. The ball screw mechanism according to claim 1, wherein an diameter-direction outer peripheral surface of at least one of said retaining pieces is formed into a concave shape. 4-5. (canceled)
 6. The ball screw mechanism of claim 1, wherein at least one of said retaining pieces is shaped as a sphere and has recesses formed therein, wherein said recesses are opposite one another.
 7. The ball screw mechanism of claim 1, wherein at least one of said retaining pieces is X-shaped in cross section.
 8. The ball screw mechanism of claim 1, wherein at least one of said retaining pieces is shaped as a cylinder and has recesses formed therein, wherein said recesses are opposite one another.
 9. The ball screw mechanism of claim 1, wherein at least one of said retaining pieces, in cross section, further comprises rounded corners. 10-14. (canceled)
 15. A ball screw mechanism comprising: a screw shaft having a spiral-shaped screw groove on its outer peripheral surface; a nut member having a spiral-shaped screw groove on its inner peripheral surface mutually facing said spiral-shaped screw groove of said screw shaft; a plurality of load balls disposed in a spiral-shaped screw groove circulation passage formed by said mutually facing screw grooves and also in a return circulation passage which is formed in said nut member and is continuously connected to said spiral-shaped screw groove circulation passage, wherein said load balls taken out from said screw groove circulation passage can be returned again into said screw groove circulation passage through said return circulation passage; a plurality of retaining pieces each interposed between every two mutually adjoining ones of said load balls, said retaining piece having two concave surfaces respectively facing said two load balls and also an outside diameter dimension in the range from 0.5 to 0.9 times the outside diameter dimension of said load ball, wherein an outer peripheral portion of each of the retaining pieces is projected in a radial direction thereof.
 16. A ball screw mechanism comprising: a screw shaft having a spiral-shaped screw groove on its outer peripheral surface; a nut member having a spiral-shaped screw groove on its inner peripheral surface mutually facing said spiral-shaped screw groove of said screw shaft; a plurality of load balls disposed in a spiral-shaped screw groove circulation passage formed by said mutually facing screw grooves and also in a return circulation passage which is formed in said nut member and is continuously connected to said spiral-shaped screw groove circulation passage, wherein said load balls taken out from said screw groove circulation passage can be returned again into said screw groove circulation passage through said return circulation passage; a plurality of retaining pieces each interposed between every two mutually adjoining ones of said load balls, said retaining piece having two concave surfaces respectively facing said two load balls and also an outside diameter dimension in the range from 0.5 to 0.9 times the outside diameter dimension of said load ball, wherein each of said retaining pieces is concave in a radial direction thereof.
 17. The ball screw mechanism according to claim 1, wherein at least one of said plurality of retaining pieces has an outside diameter dimension of 0.9 times the outside diameter dimension of said load ball.
 18. The ball screw mechanism according to claim 15, wherein at least one of said plurality of retaining pieces has an outside diameter dimension of 0.5 times the outside diameter dimension of said load ball.
 19. The ball screw mechanism according to claim 16, wherein at least one of said plurality of retaining pieces has an outside diameter dimension of either 0.5 or 0.9 times the outside diameter dimension of said load ball. 